The present invention relates to insulated-gate field effect transistors and, as an example, to MISHFETs (Metal Insulator Semiconductor Heterostructure Field Effect Transistors) and particularly to a MISHFET of GaN and the like.
Conventionally, a GaN power HFET whose cross section is shown in FIG. 7 has been known. In this GaN power HFET, an AlN buffer layer 2002, an undoped GaN channel layer 2003 and an Al0.25Ga0.75N layer 2004 are formed one on another on a SiC substrate 2001. Then, on the Al0.25Ga0.75N layer 2004, a source ohmic electrode 2005 made of a Ti/Al/Au stacked layer, and a drain ohmic electrode 2006 made of a Ti/Al/Au stacked layer are formed. A Si3N4 gate insulator 2008 is formed on the AlGaN layer 2004, and a gate 2010 is formed on the gate insulator 2008.
This GaN power HFET is an n-channel normally-ON type one, having a threshold voltage of about −6.5V. FIG. 4 shows an energy band of this GaN power HFET. FIG. 4 is an energy band view without any voltage applied thereto. The GaN power HFET has a threshold voltage which depends on the thickness and composition of the AlGaN layer 2004, the thickness of the Si3N4 layer 2008 and the work function of the gate electrode 2010. The threshold voltage of this MISHFET is substantially not more than 0 V, so that a normally-ON operation occurs. This normally-ON characteristic is shown in FIG. 6. In FIG. 6, Id denotes a drain current, Vds denotes a drain-source voltage, and Vgs denotes a gate-source voltage.
This conventional MISHFET, while having a threshold voltage of −6.5V and being a normally-ON device, is under a desire for normally-OFF operation in its many applications.
Reference Document:
V. Adivarahan, M. Gaevski, W. H. Sun, H. Fatima, A. Koudymov, S. Saygi, G. Simin, J. Yang, M. Afir Khan, A. Tarakji, M. S. Shur, and R. Gaska auctorial; Submicron Gate Si3N4/AlGaN/GaN MISHFET (Metal-Insulator-Semiconductor Heterostructure Field-Effect Transistors); IEEE Electron Device Letters (EDL), Vol. 24, No. 9, September 2003, pp. 541-543.